PURIFYING MOLTEN METAL

Abstract: The invention relates to a means of purifying molten metal in a vessel, e.g. a tundish wherein an active inclusion-removing covering layer may be used in a more cost-effective manner. Accordingly, the invention provides a method of purifying molten metal in a molten metal handling vessel (10, 10'), the vessel having an inlet region (A, A') and an outlet region (C, C') for the molten metal, characterised in that the vessel (10, 10') is divided to include a cleaning region (B, B') between the inlet region (A, A') and the outlet region (C, C'), the molten metal (21, 21') is caused to flow from the inlet region (A, A') through the cleaning region (B, B') to the outlet region (C, C') and an active inclusion-removing covering layer (23, 23') is placed on the surface (22, 22') of the molten metal (21, 21') in the cleaning region (B, B').

Description: PURIFYING MOLTEN METAL This invention relates to a method and apparatus for purifying molten metal and is particularly applicable to purifying molten steel during the continuous casting process. For convenience the invention will hereafter be described with specific reference to steel. In the continuous casting of steel, molten steel is poured from a ladle into a continuous casting mould via an intermediate vessel which acts as a constant head reservoir and is called a tundish. It is usual to treat the steel as initially produced at a stage before it is in the ladle and /or whilst it is in the ladle in order to remove undesired impurities or inclusions prior to pouring the steel into a tundish. These treatments are generally referred to collectively as secondary steel making processes, which include ladle slag removal, vacuum treatment, inert gas stirring, fluxing treatments or inert gas injection of powder reagents. Furthermore, two or more of these processes may be combined in order to achieve high quality steels having particular regard for steel cleanness. However, a proportion of impurities, whether solid or liquid, always passes from the ladle to the tundish. In addition further inclusions e.g. oxide inclusions, may result from atmospheric oxidation of the molten steel as it flows from a ladle to a tundish. There is also a tendency for the steel to pick up further impurities in the tundish and it is now common practice to attempt to remove impurities from the steel in the tundish by various means including causing the steel in its passage through the tundish to pass over and under various dams and weirs or through baffle plates. The dams weirs and baffle plates may be made of refractory material that absorbs inclusion particles. One such means of removing inclusions from molten metal in a tundish is disclosed in Patent Abstracts of Japan, vol. 5 No. 43 (M- 60) (715) and JP-A-56-001252. Molten steel is passed between a vertically- spaced stack of horizontally extending shelves of refractory material positioned between the arrival zone and outlet for molten metal. The shelves absorb inclusion particles. Another such means is disclosed in our European Patent No. 0376523 (FS 1410/ 1421) which discloses a method of purifying metal comprising passing the molten metal in a tundish past and between a vertical array of baffles located between the zone of arrival of the metal in the tundish from the ladle and an outlet from the tundish, the baffles of the array being spaced apart transversely across the tundish whereby flow of the molten meal is restricted to channels between the baffles. This prior invention is particularly useful in conjunction with constraints to move the flow of metal, usually steel, upwardly in the tundish and with the use of a surface-covering active flux layer. Such flux layers, known per se, can react with inclusions in the steel and effectively capture them and so remove them from the steel flow. It is understood that the expression "inclusions" includes indigenous and exogenous inclusions and furthermore the inclusions may be separate or in clusters. In any event, inclusions in the steel in the tundish can be harmful as a result of being deposited in the bore of the outlet nozzles including any extension thereof below the tundish extending towards the moulds or as a result of being transferred into the moulds where they may cause a deterioration in surface and internal quality and adversely affect the metallurgical properties of the steel being cast. In addition to the above-mentioned active fluxes, it is also well known to cover the surface of the molten steel in a tundish with a covering layer of powder whose purpose is to thermally insulate the steel from the atmosphere so as to reduce heat losses. It has also been proposed to use dual layers or dual purpose layers of tundish cover powders so as to obtain the combined advantages of insulation and inclusion removal. Typical insulating cover layers are based on magnesia or aluminium silicate powders whereas inclusion removing active flux layers may include, for example alumina, lime and silica. It will be appreciated that active flux layers are more expensive than purely insulation layers and there is, therefore, a need to provide a means of removing inclusions from a vessel such as a tundish that utilises expensive additive layers in a more cost effective manner. Moreover, we have found that reactive surface covering layers can cause problems in the inlet region of the vessel where turbulence and the presence of slag can cause unwanted reaction and crusting of the flux in that region. Also, where refractory stopper rods are used to open and close the outlet or outlets of the vessel, an active flux can attack the composition of the stopper rod and hence reduce its effective working life. It is, therefore, an object of the present invention to provide an improved means of purifying molten metal by the removal therefrom of unwanted inclusions and in which the above-mentioned problems are overcome or ameliorated. Accordingly, in one aspect the invention provides a method of purifying molten metal in a molten metal handling vessel, the vessel having an inlet region and an outlet region for the molten metal, characterised in that the vessel is divided to include a cleaning region between the inlet region and the outlet region, the molten metal is caused to flow from the inlet region through the cleaning region to the outlet region and an active inclusion-removing covering layer is placed on the surface of the molten metal in the cleaning region. In another aspect the invention provides a molten metal handling vessel comprising an inlet region and an outlet region: for molten metal, characterised in that the vessel has a cleaning region between the inlet and outlet regions and means to confine a covering layer of powder on the surface of the molten metal in the cleaning region. The invention is particularly suitable for the removal of inclusions from molten steel passing through a tundish and, as indicated above, will be further described with specific reference to that embodiment. The means to divide the tundish into the desired regions are preferably weirs of suitable refractory material, i.e. refractory boards, suspended from above the tundish so as to extend from above the highest level the molten steel will reach down towards the floor of the tundish and leaving a sufficient gap above the tundish floor to allow adequate flow of the molten steel. Thus in a specific embodiment, the tundish is divided into three regions by the provision of two such weirs spaced along its length and each extending across its full width. Between the two weirs is defined a central cleaning region and to one side of the cleaning region is defined the inlet region and to the other side is defined the outlet region. A reactive tundish powder, e.g. REACTOL™, can be floated on the surface of the molten steel in the cleaning region and is confined to that region by the weir which extends above the molten metal surface. The molten metal surface in the inlet and outlet regions may, if desired, be covered by a different covering layer, preferably a standard insulating cover powder. Other dams, weirs and features conventionally incorporated in a tundish may be additionally provided, as desired. For example, it is usual practice to provide an impact pad on the floor of the tundish in the inlet region to receive the stream of molten steel entering the tundish and reduce erosion of the tundish floor in that region. In a preferred embodiment a refractory dam is provided to extend upwardly from the floor of the tundish in the cleaning region so that steel entering the cleaning region by passing under the weir defining its commencement is constrained to flow upwardly by the dam so that the steel is directed towards the active covering layer. In this embodiment it is preferred, therefore, that the dam extends upwardly to a height greater than the gap between the floor of the tundish and the bottom of the weir past which the steel enters into the cleaning region. If desired, an inert gas, e.g. argon, may be bubbled upwardly through the steel in the cleaning region. This is found to encourage the inclusions to float upwardly into contact with the active covering layer. The introduction of the gas may conveniently be from within the aforementioned dam. The dam may be of any convenient shape, e.g. frusto triangular tapering upwardly and in this instance the inert gas can conveniently emerge from the upper surface of the dam. However, it may be preferred to provide an L- shaped dam with the gas emerging from the base of the "L" and sheltered downstream from immediate impact with the steel flow by the upright of the "L". In another embodiment, the dam is a perforated dam so that a proportion of the molten steel is constrained to flow upwardly by the dam while the remainder of the steel flows through the perforations through the dam. In this embodiment the source of gas is preferably positioned in the floor of the tundish on the downstream side of the perforated dam whereby steel having flowed through the dam meets the gas flowing upwardly from the gas source. The weirs to define the cleaning region (and any other dams and weirs incorporated in the tundish) preferably have a non-smooth surface, e.g. the surface may have ripples, nodules or corrugations. By this means the surface area of the weirs that comes into contact with the steel is increased, e.g. by a factor of up to 3 or more, and this can result in significantly increased inclusion "pick-up" by the surfaces of the weirs. Where nodules are provided, they could be, for example, of rounded conical or pyramidal shape or the surface could be designed to have, for example, longitudinally-extending flutes. The weirs are preferably of alumina-based refractory material as this attracts calcium silicate inclusions, which are frequently the principal non-metallic inclusions in molten steel in the tundish. It is not necessary that the entire thickness of the weir be of alumina-based refractory. An alumina-based surface over a core of cheaper refractory material may be found perfectly adequate. Embodiments of the invention are now described by way of example only with reference to the accompanying drawings in which: Figure 1 is a longitudinal cross-section through a tundish according to a first embodiment of the invention; Figure 2 is a fragmentary perspective view with parts cut away of the tundish of Figure 1 in the region of a weir; Figure 3 is a similar view to Figure 1 through a tundish according to a second embodiment of the invention; and Figure 4 is a fragmentary perspective view with parts cut away of the tundish of Figure 3 in the region of a weir. In Figure 1 , tundish 10 has a steel outer casing 1 1 lined with a relatively permanent monolithic cast lining 12 over the interior surface of which, i.e. the surface that would otherwise contact molten steel in the tundish, is an expandable refractory lining 13. The tundish is divided into three regions A, B and C by two refractory weirs 14 and 15 spaced longitudinally of the tundish and extending across its full width. Each weir 14, 15 protrudes above the uppermost rim 10A of the tundish but is of insufficient depth to reach the floor 16 of the tundish. The tundish region "B" between weirs 14 and 15 is the cleaning region of the tundish. Region "A" upstream of weir 14 is the inlet region and steel is admitted into that region from a ladle (not shown) via a conventional ladle shroud 17. An impact pad 18 is positioned on floor 16 beneath the ladle shroud 17 to receive the incoming stream of steel and to prevent undue erosion of the lining in that region. Downstream of weir 15 is the outlet region "C" and this contains an outlet 19 in the floor 16 and a stopper rod 20 by the raising and lowering of which the outlet can be opened and closed as required. Steel 21 is shown filling the tundish to a surface level 22. On the surface 22 in the cleaning region "B" is a layer of an active tundish covering powder 23, this layer being contained between weirs 14 and 15 so that it remains in the effective cleaning region only. Surface 22 of the steel in regions "A" and "C" has a covering 24 of a standard insulating cover powder. Thus the amount of active covering powder required is kept relatively low and the cleaning effectiveness of the layer optimised. Moreover the active powder layer 23 is isolated from any ladle slag carry over into the tundish. Longer sequence casts can therefor be maintained before the effectiveness of the active cover layer is significantly diminished. Upstream weir 14 extends towards floor 16 of the tundish to leave a gap of height "a" between its lower end and the floor. Downstream weir 15 similarly extends towards floor 16 to leave a gap II c _l"t between its lower end and floor 16. A dam 25 extends across the width of the tundish and upwardly from a position on floor 16 between weirs 14 and 15 to a height "b" above the floor. As shown "b" is greater than "a" and "c" so that steel flowing under weir 14 into region "B" is forced upwardly towards the surface 22 and its covering layer 23 of active powder. Argon is supplied from a suitable source (not shown) and emerges from exits 26 spaced across the dam 25 and bubbles 27 of argon gas float upwardly towards surface 22 of the steel encouraging inclusions (not shown) in the steel to float up into contact with active covering layer 23. In outlet region C of the tundish a sacrificial dam 28 extends across the width of the tundish and upwardly from the floor on the upstream side of outlet 19. This may be, for example, a slurry- formed refractory board which soon disintegrates under the influence of the molten steel, e.g. within the first 10 minutes of pouring from a ladle. Its purpose is to hold back from the outlet the first cooler steel entering the tundish so that hotter metal will flow over the dam and be first to the outlet, thereby reducing the risk of metal freezing in the outlet. As shown in Figure 1, weirs 14 and 15 are spaced "x" apart where "x" = "y" + "z", "y" being the projected distance along the floor of the tundish from the centre of the dam 25 to the facing (downstream) wall of weir 14 and "z" the similar projected distance from the centre of dam 25 to the facing (upstream) wall of weir 15. The optimum dimensions of "a", "b", "c", "x", "y" and "z" will of course vary from tundish to tundish but the average skilled man of the art will readily be able to determine appropriate dimensions for any particular circumstances so as to achieve the optimum residence time of steel in contact with active layer 23 while enabling steel throughput through the tundish to the outlet nozzle 19 to be sufficient for uninterrupted casting conditions. In Figure 2, weir 14 is shown to have on at least its surface facing weir 15 a series of generally frusto-conical nodules .29 to increase the "pick-up" of inclusions. In Figure 3 is shown a similar tundish 10' to that of Figure 1 and like parts in tundish 10' are given the same but prime numbers as for Figure 1. Thus tundish 10' is divided into three regions A', B' and C by two refractory weirs 14' and 15' as in Figure 1 and region B' is the cleaning region. As before , inlet region A' contains an impact pad 18' to receive the stream of steel and outlet region C contains an outlet 19' in floor 16'. Active tundish covering powder 23' covers surface 22' of the steel in region B' and a layer 24' of standard insulating cover powder is applied in regions A' and C. Upstream weir 14' and downstream weir 15' are positioned, as shown, similarly to their counterparts in Figure 1. A perforated dam 25' extends across the width of the tundish and upwardly from floor 16' between weirs 14' and 15'. As before, steel flowing under weir 14' into region B' is forced upwardly towards surface 22' and layer 23' of active powder. However, a proportion of the steel passes through perforations 25A in dam 25'. Argon is supplied from a suitable source (not shown) and emerges from a bubble brick 30 extending across the floor 16' of the tundish immediately downstream of the dam 25'. Bubbles (not shown) of argon gas float upwardly towards surface 22' of the steel encouraging inclusions (not shown) in the steel to float up into contact with active covering layer 23'. (It will be appreciated that, although not shown, the tundish 10' of Figure 3 will be provided with closure means such as a stopper rod for outlet 19'.). As shown in Figure 4, weir 14' is provided with nodules 29' of frusto-pyramidal shape to increase inclusion pick-up. Weir 15' is similarly provided with nodules. It will be appreciated that various embodiments of the invention as described above can be varied without departing from its scope and spirit. As previously indicated, the weirs may be provided with surfaces of different shape to that disclosed. The outlet flow control device may be a rotary slide valve, a slide gate valve or a simple metering nozzle. The impact pad may be of domed or other desired shape.

Claims: CLAIMS 1. A method of purifying molten metal in a molten metal handling vessel (10, 10'), the vessel having an inlet region (A, A') and an outlet region (C, C) for the molten metal, characterised in that the vessel (10, 10') is divided to include a cleaning region (B, B') between the inlet region (A, A') and the outlet region (C, C), the molten metal (21, 21') is caused to flow from the inlet region (A, A') through the cleaning region (B, B') to the outlet region (C, C) and an active inclusion- removing covering layer (23, 23') is placed on the surface (22, 22') of the molten metal (21, 21') in the cleaning region (B, B'). 2. A method according to Claim 1, characterised in that the vessel ( 10, 10') is provided with two weirs ( 14, 14' and 15, 15') spaced along its length and extending across its full width to define the cleaning region (B, B') between the two weirs. 3. A method according to Claim 2, characterised in that the weirs ( 14, 14' and 15, 15') are refractory boards extending from above the highest level the molten metal reaches in the vessel (10, 10') down towards the floor (16, 16') of the vessel (10, 10') leaving a sufficient gap (a, a' and c, c') above the floor (16, 16') to allow adequate flow of the molten metal. 4. A method according to Claim 2 or 3, characterised in that the covering layer (23, 23') of active inclusion-removing material is confined between the weirs (14, 15 and 14', 15'). 5. A method according to any preceding claim, characterised in that the surface (22, 22') of the molten metal (21, 21') in the inlet (A, A') and outlet (C, C) regions is covered by a standard insulating layer material. 6. A method according to any preceding claim, characterised in that a refractory dam (25, 25') is provided extending upwardly from the floor (16, 16') of the vessel (10, 10') so that metal flowing into the cleaning region (B, B') is constrained by the dam to flow upwardly 7. A method according to Claim 6, characterised in that an inert gas is bubbled upwardly through the molten metal (21, 21') in the cleaning region (B, B'). 8. A method according to Claim 7, characterised in that the gas emerges from the upper surface of the dam (25). 9. A method according to Claim 7, characterised in that the dam (25') is provided with perforations (25A) through which a proportion of the metal flows and the gas emerges from a source (30) on the floor (16') of the vessel (10') downstream of the dam (25'). 10. A molten metal handling vessel (10, 10') comprising an inlet region (A, A') and an outlet region (C, C) for molten metal (21, 21'), characterised in that the vessel (10, 10') has a cleaning region (B, B') between the inlet (A, A') and outlet (C, C) regions and means (14, 14 * and 15, 15') to confine a covering layer of powder (23, 23') on the surface (22, 22') of the molten metal in the cleaning region (B, B'). 11. A molten metal handling vessel according to Claim 10, characterised in that the means (14, 14' and 15, 15') is a pair of refractory boards spaced along the length and extending across the width of the vessel (10, 10') to define the cleaning region (B, B') between them. 12. A molten metal handling vessel according to Claim 11, characterised in that the boards (14, 14' and 15, 15') extend from a height in the vessel (10, 10') above the highest level the molten metal (21, 21') is to reach and down towards the floor (16, 16') of the vessel (10, 10') leaving a gap (a, a' and c, c') above the floor for passage of molten metal. 13. A molten metal handling vessel according to Claim 10, 11 or 12, characterised in that a dam (25, 25') extends upwardly from the floor (16, 16') of the vessel (10, 10') in the cleaning region (B, B'). 14. A molten metal handling vessel according to any one of Claims 10 to 13, characterised in that a source (26, 30) of inert gas is provided adjacent the floor (16, 16') of the vessel (10, 10'). 15. A molten metal handling vessel according to Claim 14, characterised in that the source of gas (26) outlets from the dam (25). 16. A molten metal handling vessel according to Claim 15, characterised in that the dam is "L"-shaped and the gas outlets from the base of the "L" on the downstream side of the dam. 17. A molten metal handling vessel according to Claim 13 or 14, characterised in that the dam (25') has perforations (25A) to allow molten metal to pass through. 18. A molten metal handling vessel according to Claim 17, characterised in that the source of gas (30) outlets from the floor (16') of the vessel (10') downstream of the dam (25'). 19. A molten metal handling vessel according to any one of Claims 11 to 18, characterised in that the refractory boards (14, 14'. and 15, 15') are provided with nodules (29, 29') on their surfaces facing the cleaning region (B, B'). 20. A molten metal handling vessel according to any one of Claims 10 to 19, characterised in that it is a tundish (10, 10') having inlet pouring means (17), an impact pad (18, 18') to receive the incoming metal and an outlet (19') closable by a stopper rod (20) or a slide gate valve.

Inventors: CLARK MICHAEL ROBERT, SUTTON DENNIS, CLARK, MICHAEL, ROBERT, SUTTON, DENNIS
Applicants: CLARK MICHAEL ROBERT, FOSECO INT, SUTTON DENNIS, CLARK, MICHAEL, ROBERT, FOSECO INTERNATIONAL LIMITED, SUTTON, DENNIS
Application year: 1994
Main classification: B22D11
IPC codes: B22D11, C21C7, C22B9